This invention relates to a coupler apparatus for use in combination with a noninvasive ophthalmological method for reshaping the anterior surface of the cornea in order to achieve emmetropia (i.e., normal vision characterized by the absence of ocular refractive error; the emmetropic eye focuses parallel rays of light onto the retina to produce a clear image). The method of the invention uses light energy to induce thermal changes to the collagen in the stromal portion of the cornea in order to physically reorganize the stromal collagen to produce the desired reshaping of the cornea. The method is described in commonly-assigned, copending U.S. patent application Ser. No. 556,886 filed Jul. 23, 1990 which is hereby incorporated by reference as if fully set forth herein.
The apparatus of this invention is referred to as a coupler based on its utility, which is to couple a heat and light energy source to the cornea surface. It is made of a material which is substantially transparent to the light energy used to reshape the cornea. In this fashion the coupler acts as a heat sink. The coupler conducts heat from the anterior portion of the cornea during the heating of the stroma. The coupler has a corneal engaging surface so that the coupler is positioned on the anterior surface of the cornea. This corneal engaging surface has a radius of curvature which approximates the desired emmetropic shape of the cornea following the heating of the stroma and rearrangement of the collagen.
Today there are over 100 million people in the United States alone who wear eyeglasses or contact lenses to correct ocular refractive errors. The most common ocular refractive errors include myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. In myopia, the refractive power of the eye is excessive meaning that parallel rays of light are focused in front of the retina producing a blurred image. Myopic vision can be modified, reduced or corrected by adding a spherical concave lens of the correct spherical curvature in front of the eye or by flattening the cornea axisymmetrically around the visual axis to reduce its refractive power.
In hyperopia (also termed hypermetropia), the refractive power of the eye is deficient meaning that parallel rays of light are focused behind the retina producing a blurred image. Hyperopic vision can be modified, reduced or corrected by adding a spherical convex lens of the correct spherical curvature in front of the eye or by steepening the cornea axisymmetrically around the visual axis to increase its refractive power.
In astigmatism, the refractive power of the eye is unequal in all meridians meaning that parallel rays of light are focused differently along different meridians producing a blurred image. Astigmatic vision can be modified, reduced or corrected by adding a non-spherical lens of the correct cylindrical curvatures along various meridians in front of the eye or by flattening and/or steepening the cornea with the correct cylindrical curvatures to compensate for refractive errors along various meridians.
Current widely used devices or methods for correcting ocular refractive errors include eyeglasses, contact lenses and refractive surgery such as radial keratotomy. Eyeglasses and contact lenses may be inconvenient, difficult to wear or impediments in daily activities.
Refractive surgery procedures offer an alternative to eyeglasses and contact lenses but these procedures may be difficult to control in order to achieve accurate refractive corrections. Radial keratotomy is a refractive surgical procedure designed to correct myopia. This technique involves making a series of deep, radial incisions in the cornea with a pattern that resembles the spokes of a bicycle wheel. The incisions themselves do not cross the center of the cornea, the central optic zone. The series of symmetrical cuts flatten the cornea.
Significant percentages of patients who have been subjected to radial keratotomy experience overcorrection, undercorrection or induced astigmatism. Radial keratotomy patients may also suffer from side effects and postoperative complications such as fluctuating refraction, glare, reduced night vision, photophobia, endothelial cell loss, and corneal infection. Another postoperative complication of radial keratotomy is permanent weakening of the cornea due to the fact that the technique requires deep incisions that heal quite slowly. Trauma to the eyes may result in the rupture of the incisions leading to catastrophic loss of the cornea in some cases.
Another method of refractive surgery is laser keratomileusis (i.e., carving the cornea by application of laser energy) also termed laser refractive keratectomy or photorefractive keratectomy. This method of refractive surgery is currently being used in clinical trials in man to correct refractive errors. This technique employs the use of a laser that emits ultraviolet light, typically an argon fluoride excimer laser that operates at a wavelength of 193 nanometers. The laser light causes a breakdown of intramolecular bonds resulting in ablation of tissue by photodecomposition. The shape of the cornea is changed by selectively ablating material in the cornea thus xe2x80x9ccarvingxe2x80x9d the anterior corneal surface into a new shape. U.S. Pat. No. 4,665,913 discloses one technique of photorefractive keratectomy.
As is the case for other forms of refractive surgery, photorefractive keratectomy may lead to inadequate refractive corrections and to undesirable side effects. Particularly troublesome is the postoperative complication associated with corneal wound repair, a process that tends to xe2x80x9cfill inxe2x80x9d the ablated cornea volume with a combination of epithelial and stromal tissues. This process in the human cornea is sometimes referred to as a wound-healing response. There are also concerns about the potential phototoxic effect of ultraviolet light generated by corneal tissue fluorescence and the potential toxic effect of molecular ablation products present in the photoablation plume.
Another method of refractive surgery is intrastromal photorefractive keratoplasty. In this technique a laser beam is focussed inside the corneal stromal tissue to modify tissue either by photoablation or by a change in the tissue""s viscoelastic properties. U.S. Pat. No. 4,907,586 discloses one such method for optical laser surgery. It is not clear when the supporting work for this patent was performed. The wavelengths of the laser beams to be used are specified to be 526 nanometers, 1.053 microns, or 2.94 microns. Some of these wavelengths (526 nanometers and 1.053 microns) are transmitted, at least in part, through the cornea possibly causing damage to the retina. If laser induced optical breakdown (i.e., laser induced plasma formation) is used to increase the absorption of these wavelengths, the hot plasma will reradiate light with a broad wavelength distribution that includes phototoxic light in the ultraviolet spectral region. The final wavelength (2.94 microns) specified in U.S. Pat. No. 4,907,586 is absorbed completely in the anterior portion (particularly, the epithelium) of the cornea [G. L. Valderrama, et al., SPTE Proceedings, Vol. 1064, 135-145 (1989)] so that it cannot produce intrastromal tissue modification. The alleged intrastromal photorefractive keratoplasty method is unworkable at some wavelengths and undesirable at other wavelengths because there may be severe damage caused to ocular structures.
Thermokeratoplasty is another method that has been used to reshape the cornea. This is done by the application of heat to the cornea. Corneal stromal collagen shrinks when heated to a temperature of 55xc2x0 to 58xc2x0 C., without the destruction of the tissue. The stroma is the central, thickest layer of the cornea and consists mainly of collagen fibers. If the pattern of shrinkage is properly selected the resulting change in the stress field and mechanical properties caused by the shrunken collagen fibers can be used to reshape the cornea.
The original thermokeratoplasty technique used was the application of a heated probe to the cornea leading to conductive heating of the stroma. However, the direct application of a heated probe is uncontrolled and unavoidably destructive. This technique caused thermal destruction of the epithelium as well as Bowman""s membrane, the important tissue layer immediately underlying the epithelium. Some patients treated with this technique also showed damage to the deeper corneal stroma and endothelium. Additionally, this technique often involved inadequate refractive correction. Others have attempted the correction of hyperopia by using heated needles to burn a series of craters into the cornea. Recently the hot needles have been replaced with a laser in an effort to produce a more controlled thermal deposition. Severe damage to the corneal tissue still occurs and makes these procedures too undesirable for most ophthalmologists to recommend to their patients.
Another method of themokeratoplasty involves microwave heating of the corneal stromal collagen. Microwave energy can be deposited deeply within the corneal stroma. U.S. Pat. No. 4,881,543 discloses one method and apparatus for heating the central stroma of the cornea with microwave electromagnetic energy to the shrinking temperature of the collagen while circulating a cool fluid over the anterior surface of the cornea. However, microwave thermokeratoplasty procedures do not provide the spatial and temporal resolution and control required to perform accurate cornea reshaping without excessive thermal damage to cornea structures.
There is still a need for a method of reshaping the cornea that is safe, effective, and dependable. The apparatus of this invention, used in combination with a method employing light energy to safely reshape the cornea by controlled heating of the stromal collagen, offers a significant advancement to the field of cornea reshaping.
This invention relates to an apparatus for reshaping the curvature of the cornea of the human eye. The reshaping is intended to correct ocular refractive errors. The means for reshaping is by a controlled heating of the anterior portion of the stroma of the eye. The apparatus of this invention is a coupler which is typically made from infrasil quartz, calcium fluoride, sapphire, diamond or combinations thereof and which is configured to be removably attached to the anterior most surface of the cornea.
The coupler device is highly transparent to the preferred wavelengths of functionally effective light energy. The light energy is typically supplied in the range of 2.4 to 2.67 microns for a hydrogen fluoride chemical laser or 1.90 to 2.02 microns for a thulium doped laser. The function of the coupler is to act as a heat sink and thermostat; a template for the cornea; a positioner and restrainer for the eye; and a mask during the reshaping procedure.
The coupler device has a corneal engaging surface which is functionally sized to be removably attached to the anterior surface of the cornea. In the most preferred embodiment, the corneal engaging surface has the desired emmetropic shape for corrected vision. The radius of curvature of the cornea must be calculated so that a similar curvature of the corneal engaging surface of the coupler device can be used.
The coupler device of this invention also includes means for retaining the coupler on the corneal surface during the treatment procedure. The coupler device further includes means for immobilizing the eye to ensure that the treatment procedure avoids the central optic zone of the eye. The most preferred retaining means and immobilizing means is an annular suction ring.
The coupler device of this invention also functions as a mask to provide a specific pattern of light energy application to the cornea. The specific geometry of the pattern of the masking feature of the coupling device is important to the functioning of the corrective method. In this fashion different corrections and different degrees of correction can be encompassed within a single coupler device with interchangeable or interusable masking means. The masking means is typically found on the surface of the coupler opposite the corneal engaging surface although any of the other surfaces may be functionally effective for masking purposes.
It is important to note that the corneal correction takes place without initiating a wound-healing response. The initiation of a wound-healing response introduces significant error into the treatment and must be avoided. The coupler device of this invention facilitates the protection of the stromal collagen from temperatures and conditions sufficient to initiate such a wound healing response through its various functional capabilities.
Although in the preferred embodiment of this invention a laser light is used in combination with the coupler, other known means for transmitting light energy can also be used. For instance, a fiber optic material can be used to control the light source as well as other light controlling means that are well known to those of ordinary skill in the art.
In the preferred embodiment of this invention, a corneal topography measuring device is used to determine the change in the shape of the cornea of the eye, such devices include a surgical keratometer or photo keratoscope. Likewise, in the preferred embodiment, means are provided for viewing the cornea of the patient""s eye, i.e. a surgical microscope, and a refraction measurement system is used to determine the total refraction of all components of the eye. In supplying light energy it is important to observe criteria of pattern application, irradiance levels, duration of application, and wave form. At all times the central optic zone of the eye is protected or shielded from radiation. The avoidance of the central optic zone permits avoidance of the risk of undesirable, long term, adverse effects on sight.
The coupler acts as a heat sink and thermostat by conducting heat away from the anterior portions of the cornea during the heating of the stroma. It is important that the section of the cornea referred to as Bowman""s membrane be kept below the stromal collagen shrinkage temperature to avoid damage to Bowman""s membrane during the procedure. Bowman""s membrane controls the regeneration of the epithelium layer of the cornea, thus the importance of not damaging the membrane with the light energy used to heat the stroma.
The coupler acts as a template for the cornea by having a corneal engaging surface positioned on the anterior surface of the cornea. The corneal engaging surface has a radius of curvature which approximates the desired emmetropic shape of the anterior of the cornea. During the heating process, the cornea is reshaped, and the shape contours to the corneal engaging surface which is the desired emmetropic shape.
The coupler also acts as a positioner and restrainer for the eye by attaching to the eye via attachment means, i.e., an annular suction ring. This attachment allows the optical surgeon to position the eye as desired and also permits holding the eye in a fixed position during the procedure. A vacuum of approximately 10 mm Hg is used to attach the coupler to the eye. In the preferred embodiment of this invention the coupler is attached to a stable platform to insure proper alignment of the light source, coupler and eye at the time of irradiance. In the most preferred embodiment of this invention the stable platform is an articulated arm.
And finally, the coupler acts as a mask to prevent accidental exposure of the central optic zone to any light energy during the irradiance procedure. The masking feature likewise permits selection and control of the geometric pattern for application of light energy. Such control permits specific procedures to be defined and used for specific optical corrections.